Each microscopic particulate of dust in a fluid is electrically charged and has an electrical potential at the interface with the fluid. A known method of removing such dust particulates relies on the use of a zeta potential-applied filter element consisting of a filter coated with powder of barium titanate which has a natural potential when no external potential is applied; therefore, the filter itself has a potential. However, as this filter element is used, the coating material gradually flows out, so that the potential of the filter is gradually lost. As a result, the effective time of the filter is short. Consequently, it must be replaced frequently. This increases the cost.
In view of the foregoing situations, the present applicant has proposed a new apparatus in Japanese Utility Model Laid-Open No. 989113/1991. In particular, an inner cylindrical electrode is mounted inside an outer cylindrical electrode which acts also as a container of the body of the apparatus. The inner and outer electrodes are placed in the same potential and mounted in a coaxial relation to each other. A space through which a liquid to be processed circulates is formed between both electrodes. A filter having a porous metallic plate disposed directly on its outer surface is mounted in the space. The porous metallic plate is directly electrically charged to attract the interface potential (or, zeta potential) of each impurity particle in the liquid contained among the outer electrode, the inner electrode, and the porous plate by the Coulomb force so that the impurity particles may coagulate and form coarser particles. These coarser impurity particles are caught by the meshes of the filter mounted behind the porous plate to filter out the impurities in the fluid. In this apparatus, the potential for neutralizing the zeta potential is applied from the outside and so the potential on the surface of the filter will not be lost even after prolonged use of the apparatus.
However, in this apparatus, since the fluid is introduced into the filter via the porous metallic plate, the amount of inflow of the fluid into the filter depends on the porosity of the porous plate. Therefore, a limitation is imposed on the processed amount. Also, in this apparatus, the potential is applied only to the surface of the filter and so radially deep portions of the filter are not sufficiently electrically charged. Hence, the present situation is that impurity particles are collected at low efficiency in radially deep portions of the filter.
Furthermore, in the above-described apparatus, the radially deep portions of the filter are not electrically charged sufficiently and, therefore, the apparatus can be applied only to relatively large particles having sizes of about 10 microns. To coagulate particulates having smaller sizes, it is necessary to circulate the fluid through the filter many times. Consequently, the processing time is long.
In addition, where the above-described apparatus is used, if the efficiency at which impurity particles are coagulated to form coarser particles should be enhanced, the only one usable method is to increase the voltage for charging the filter. Especially in the DC electrical charging method, if the applied voltage is increased, the charging electrode undergoes electrolytic corrosion to thereby ionize the metallic electrode. For example, where an electrode made of a stainless steel is used, hexavalent chromium is produced. Therefore, it is difficult to process the wastewater. In consequence, limitations have been placed on increase in the applied voltage.